1. Field of the Invention
This invention relates generally to metal finishing compounds and, more particularly, to a nonchelating metal finishing compound that is adapted for use in a mass finishing apparatus.
2. Description of the Related Art
Finishing processes for the manufacture of metal parts or components are often carried out in two stages: deburring followed by burnishing. In the deburring stage, the metal parts are placed in a mass finishing apparatus such as a vibratory, barrel, or centrifugal finisher with an appropriate media that typically comprises substantially solid, irregularly shaped members that impinge the metal parts to remove rough corners and edges as well as manufacturing waste adhering to the parts.
In the burnishing stage, the metal components are retained in the vibratory, barrel or centrifugal finisher and are mixed with an appropriate burnishing compound. The burnishing compound generally comprises case hardened steel or stainless steel balls, cones, ball cones, eggs or other members of appropriate shape (media) and a lubricating agent that is continuously fed into and drained from the finisher. As the metal components vibrate and forcibly brush past the lubricated media, manufacturing oils and the like are removed and a bright, lustrous, polished appearance is imparted to the surface of the metal components. Different formulations for the burnishing compound are known, examples of which are disclosed in U.S. Pat. No. 4,235,051, issued Nov. 25, 1980 to Spekman, Jr. and assigned to Fremont Industries, Inc. and U.S. Pat. No. 4,367,092, issued Jan. 4, 1983 to Unzens and assigned to Roto-Finish Company, Inc.
Typically, the lubricating agent is maintained at an acidic pH to inhibit the formation of iron hydroxides, the iron resulting from the steel or stainless steel media. The acidic pH also inhibits the formation of other metal hydroxides, the particular hydroxide, of course, depending on the composition of the metal part being finished. The formation of metal hydroxides during burnishing is considered undesirable because it tends to cloud or obscure the lubricating agent, thereby detracting from its utility and shortening its effective life. Perhaps even more importantly, the metal hydroxides can be transferred to the surfaces of the metal parts being finished, thereby deteriorating the bright, polished, lustrous appearance that the burnishing stage is intended to impart to the finished metal components. To further inhibit the formation of metal hydroxides, chelating agents are often incorporated into the lubricating agent. Alternatively, a lubricant that is inherently chelating is employed or an acid having inherent chelating properties is used to acidify the lubricating agent. The chelating agents complex the iron or other heavy metal ions and form stable, soluble organo-metallic complexes that remain in solution without detracting from the burnishing process. Most commercially available barrel or vibratory burnishing agents presently known, including those identified in the above-referenced patents to Spekman, Jr. and Unzens, contain a high proportion of chelating agents.
However, increasingly stringent environmental regulations on the federal, state and municipal levels have made it more difficult if not illegal to dispose aqueous industrial waste having high concentrations of heavy metals into the public sewer system. The maximum allowable concentration of heavy metal salts in industrial waste streams is declining in response to enhanced environmental concerns. Copper salts, as one example, are particularly problematic because they are considered toxic even at relatively low concentrations. Normally, heavy metals can be removed from an industrial effluent stream by adjusting the pH of the stream to a range of approximately 8.5-9.5. Within this pH range, most heavy metals can be precipitated as highly insoluble hydroxides and thereafter filtered from the effluent, although a polymeric flocculating agent may be required to induce the formation of a coarse, readily filterable precipitate. However, the organo-metallic complexes that are formed in the presence of chelating agents are highly soluble and stable and do not readily form filterable precipitates under the industrial effluent treatment conditions presently utilized. As a result, the organo-metallic complexes or chelates pass unchanged through the effluent treatment and enter the public sewer system.
As noted hereinabove in the Background, it is desirable to maintain the lubricating agent at an acidic pH to inhibit the formation of metal, especially iron, hydroxides. However, there is a countervailing problem in that a low pH environment encourages the formation of corrosion. In a typical commercial burnishing operation, the mass finishing apparatus is drained of the lubricating agent at the end of each day as well as prior to longer periods of interruption, such as weekends and holidays. It is common for the case-hardened steel or stainless steel media to be retained in the mass finishing apparatus until it is needed for the next burnishing cycle. Exposure of the damp, lubricating agent coated media to the oxygen rich air provides an environment in which the rapid formation of corrosion on the media is promoted. The corrosion not only deteriorates and shortens the useful life of the burnishing media but can be imparted to the metal parts finished therewith, thereby further detracting from the bright, polished and lustrous appearance with which the metal parts are to be provided. Accordingly, lubricating agents are sometimes formulated with corrosion inhibitors. However, the most widely known corrosion inhibitors are also excellent chelating compounds. For example, the corrosion inhibiting compounds disclosed in the above-identified patent to Unzens are chelating agents. As described hereinabove, it is undesirable for the burnishing agent to include a chelating compound as such will complex the heavy metal ions resulting from the steel media or the parts being finished therewith.
Thus, there is a considerable need for a burnishing agent that imparts a bright, polished, lustrous appearance to the metal components being treated but which is free from compounds that tend to chelate or complex the metal ions that result from the steel media or the metal of the parts being finished. With such a burnishing agent, heavy metals resulting from the media and the parts being finished can be easily precipitated and filtered from the industrial waste effluent as insoluble hydroxides by adjusting the pH of the waste stream.
There is also considerable need for a burnishing agent that incorporates a nonchelating corrosion inhibitor. Such a burnishing agent would be particularly useful because it would resist the formation of corrosion on the burnishing media when the media is exposed to the acidic lubricating agents that are commonly employed.